1. Field of the Invention
The invention relates generally to sealing devices for reciprocating pumps. More particularly, the invention relates to a sealing device with an anti-extrusion ring that reduces the extrusion of an elastomeric sealing section of the device through the gap between the piston flange and the pump cylinder by controlling the expansion of the anti-extrusion ring, even after the flange and the cylinder are worn. The invention is particularly applicable to pumps in abrasive services, such as pumping the drilling mud in oil fields at pressures of up to 4,000 psi or higher.
2. Description of the Prior Art
In the oil well drilling art, an oil well being drilled is constantly flooded with a mud or clay slurry which is pumped at pressures up to 4,000 psi. These pumps are commonly reciprocating pumps in which a flanged piston moves within a cylinder. To permit reciprocation, the flange of the piston is typically slightly smaller in diameter than the cylinder. The gap between the flange and the cylinder wall is sealed by an elastomeric seal which is designed for easy replacement because of the wear which it suffers in use.
Elastomeric seals function by substantially sealing against the piston liner so that there is only a very minute leakage of the working fluid through the seal in response to the hydrostatic pressure of the contained fluid. Because under pressure the elastomeric seal actually behaves more like a fluid, the hydrostatic pressure transmitted through it causes the sealing material to extrude into the gap between the cylinder liner and the piston flange, at which point most of the sealing action occurs. However, as the piston reciprocates, the elastomeric material in the gap wears and erodes. This wearing and breaking off occur until the seal is no longer able to seal the fluid in the cylinder, at which time the seal must be replaced. Extrusion of elastomeric seals in mud pumps is discussed in detail in an article in Petroleum Engineer International, October, 1981, entitled "Mud Pump Failure Analysis, Part 4--Extrusion, The Primary Cause of Piston Failure."
Several devices have been proposed that would lessen extrusion and, consequently, wear of an elastomeric sealing section of a sealing device in a reciprocating pump, such as a mud pump. A common expedient is to include fabric or similar reinforcement in the heel section of the elastomeric seal to reduce its tendency to extrude into the gap between the piston flange and cylinder wall. The patent literature has suggested other techinques to reduce extrusion. For example, U.S. Pat. No. 2,926,976 to Bowerman et al. discloses a pump sealing device with an elastomeric sealing section and an anti-extrusion ring; the elastomeric sealing section is located with one side in contact with the working fluid and is bonded to a rubber-impregnated fabric section, which serves as an anti-extrusion section; the rubber-impregnated fabric section is bonded to a base ring, which seats against the piston flange. The base ring is a rigid, nonexpandible ring that has a diameter equal to the diameter of the rubber-impregnated fabric and, thereby, serves to reduce the area available for extrusion, but does not eliminate it.
U.S. Pat. No. 3,521,893 to Josephson discloses a pump sealing device with an elastomeric sealing ring and an anti-extrusion ring where the elastomeric sealing ring has a groove at one end and an anti-extrusion ring is located in the groove but not bonded to the elastomeric sealing ring. The anti-extrusion ring has a radially outward, reduced-area portion that is forced into intimate contact with the cylinder by the hydrostatic pressure generated during operation of the pump. The size of the reduced-area portion and the material from which the anti-extrusion ring is made are selected so that the yield strength of the anti-extrusion ring is exceeded to enable the anti-extrusion ring to make intimate contact with the cylinder wall. It appears that Josephson intended for the region of the anti-extrusion ring adjacent the cylinder surface to deform plastically under the influence of the pressure of the contained fluid. This system, however, does not suggest bonding the anti-extrusion ring to the elastomeric sealing material. Because of the absence of bonding, expansion of the anti-extrusion ring is uncontrolled, leading to excessive wear when employed in mud pumps.
U.S. Pat. No. 4,281,590 to Weaver discloses a pump sealing device with an elastomeric sealing section and an anti-extrusion ring where the elastomeric sealing section is bonded to the anti-extrusion ring. The anti-extrusion ring is "L" shaped or "U" shaped. The anti-extrusion ring expands radially and elastically under the influence of the hydrostatic pressure generated during operation of the pump to make intimate contact with the cylinder. This patent suggests that plastic anti-extrusion rings be used.
Both plastic and metal anti-extrusion rings are suggested by the prior art. Plastic anti-extrusion rings do not cause as much wear on the cylinder as metal anti-extrusion rings. However, if a plastic anti-extrusion ring is used in a mud pump, which operates at high pressures and high temperatures, the plastic will be relatively subject to extrusion. Consequently, the seal wear problem has not been eliminated by the use of plastic anti-extrusion rings in mud pumps since both the plastic anti-extrusion ring and the elastomeric sealing section extrude.
If a split metal anti-extrusion ring which is not bonded to the elastomeric seal is used in a mud pump, the ring will expand radially, and independently of the elastomeric sealing material, in response to hydrostatic pressure transmitted by the seal. This will force the ring against the cylinder wall with a large force per unit area by the action of the hydrostatic pressure in the working fluid. As a result, both the ring and the cylinder will wear very quickly because of the large abrasion forces that arise due to the tight metal-to-metal contact, which is caused by the high pressures, and the abrasive fluids encountered in mud pump operations.